The present invention relates to an optical scanning apparatus and system for scanning a surface to be scanned optically, and more particularly, to an electronic photographic printer, a scanner for scanning a surface of a document to read it as an image, and a photographic exposure apparatus for scanning a photosensitive surface to expose it.
In the optically scanning technique, the scanning is made by changing the direction of light. For example, in an electronic photographic apparatus, laser light (optical beam) concentrated into a beam is irradiated on a surface of a rotating mirror to vary the direction of the optical beam for scanning. There occurs a disadvantage that a scanning speed of the optical beam and a shape of a light spot formed by incidence of the optical beam on a surface of an object to be scanned, for example, a photosensitive body are varied. More particularly, there occurs a phenomenon (distortion aberration) that intervals (pitch) of the spots formed on the surface of the photosensitive body in the scanning direction are varied at a middle portion and a peripheral portion thereof and a phenomenon (curvature of field) that a position on which the optical beam is focused is shifted at the peripheral portion on the photosensitive body even if the optical beam is focused at the middle portion on the sensitive body.
FIG. 18 illustrates a principle of occurrence of the distortion aberration and the curvature of field. When a reflection position of the optical beam on a rotating mirror is O and a position on an object to be scanned is A-A', a relation of a width Dy.sub.0 on A-A' upon scanning by the optical beam by .DELTA..theta. in the direction of the point O from A', that is, in the direction of .theta.=0 and a width Dy.sub.1 on A-A' in the direction of .theta.=.theta. is geometrically calculated by the following equation: EQU Dy.sub.1 =Dy.sub.0 /cos .theta. (1)
Thus, Dy.sub.1 is larger than Dy.sub.0. In other words, the equation (1) means that the width on the object to be scanned is varied at irregular intervals when the scanning angle is varied at regular intervals. This is the distortion aberration.
When light reflected from the point O is focused on the object on A-A' in the direction of .theta.=0, the position of the focal point of the light is moved along a circular arc of A'-B when the angle .theta. is changed and according the light is not focused on the surface A-A'. Such deviation of the focal point from the surface A-A' is named the curvature of field.
In the prior art, in order to correct the above distortion, at least two lenses, so-called f.theta. lenses, disposed between the rotating mirror and the photosensitive body are used.
Further, in order to reduce the distortion, if the scanning angle of the rotating mirror is made small, a necessary scanning width can not be ensured and hence it is necessary to increase a distance between the rotating mirror and the object to be scanned. In any case, there is a problem that an optical scanning apparatus itself is made large in size. In order to solve this problem, there is a technique that light is reflected plural times to lengthen an effective length of a light path as compared with an apparent length thereof. As this technique, there are known a technique of using a plurality of mirrors for reflecting light in the air plural times and a technique of using an optical block for the same purpose. A representative prior art relating to the former is described in Japanese Patent Unexamined Publication No. 59-198417 corresponding to U.S. Pat. No. 4,966,446. Representative prior art relating to the latter is described in Japanese Patent Unexamined Publication Nos. 2-301715 and 63-71824 and U.S. Pat. Nos. 4,867,547, 4,730,882 and 4,239,337.
On the other hand, there is a case where even a scanner having an opposite path to the optical path extending from the scanning mirror to the object to be scanned as described above, for example, a reading scanner having a reduced optical system utilizes an f.theta. lens disposed between an object to be read and a reading sensor for the same purpose and uses a plurality of mirrors to reflect light plural times, so that the apparatus is made small in size.
The above prior art has the following problems.
(1) When the expensive f.theta. lens is merely adopted only for correction of the optical distortion, costs of not only an optical scanning system and a device using this optical scanning system but also an optical scanning apparatus having the device mounted thereon are influenced and a low cost optical scanning apparatus can not be realized any more.
(2) In order to make small the optical system, even if the mirror or the optical block for reflecting light plural times is adopted, the expensive f.theta. lens is indispensable for reduction of the distortion.